On May 21, 2010, at 5:41 AM, Andrew Wood wrote:
>
> Hi
>
> Im new to PIC and electronics in general, having come from a software
> background. Im looking to design something which can sample 90 wires  
> for
> the presence or absence of a DC voltage (50v, but can be taken down to
> say 5v with a voltage divider if needed). It then needs to store a bit
> in memory for each wire indicating if the wire was live or not and  
> then
> interface to a computer over USB so the computer can read the bits.
>
> Having done some research it seems to me that a PIC based system is
> probably the best way to go, but I was wondering whats the best way to
> interface the 90 wires to the PIC? All the example projects I can find
> so far interface to a much smaller number of wires.
>
> Thanks
> Andrew


Consider isolating the entire measuring circuit.
You can construct or buy a VFC ( voltage to frequency converter ).
      A VFC can be made from a PIC or logic gates or op amps )
Have the VFC look at the 90 voltages using addressable analog  
multiplexers.
The address to the multiplexers can come from logic counters or shift  
registers
driven by a PIC.

The isolated output can be "breaK" condition ( indicating the zeroeth  
time slot ) followed by 90 time slots.
Each time slot would feature a frequency indicating the state of one  
of the wires.

I have some schematics if you are interested.

Gus

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